Active electronically scanned arrays (AESA, also known as phased array antennas) require phase shift for beam pointing. AESAs typically require amplitude taper to achieve low side lobe levels (SLLs). Currently, quantization in the power output of power amplifier modules creates aperture amplitude excitation errors. Precise aperture phase and amplitude control are required for high performance AESAs. Errors in phase or amplitude cause corruption in the radiation pattern.
There has been a long-felt and persistent need, which has been unsolved by others, for removing periodic (e.g., deterministic) phase error, random amplitude error, and other phase errors from the phase shifters that are required to steer radiation beams in AESAs. Peak and root mean square phase and amplitude errors degrade AESA system parameters. For example, periodic errors on the aperture cause undesirable peak periodic side lobe levels, and root mean square random errors increase overall average side lobe level noise. Further, amplitude quantization in the aperture excitation produces undesirable deterministic peak side lobe levels in the AESA's far field radiation pattern.
Currently, transmit/receive active electronically scanned arrays (AESAs) utilize uniform illumination in transmit mode with identical power amplifier modules operated in saturation (i.e., conversion of direct current (DC) to maximum radio frequency (RF) power) to achieve the currently best available power added efficiency, and the identical power amplifiers are operated in the same manner throughout the aperture. AESAs, however, would benefit from low side lobe level transmit patterns. For example, modern radar systems require 2-way side lobe levels of −50 dBc (i.e., decibels relative to the carrier), where 2-way side lobe level (in dBc) equals receive side lobe levels plus transmit side lobe levels. Currently, uniformly illuminated rectangular (e.g., square or rectangular) contoured transmit apertures only have −13.5 dB side lobe levels in the principal E and H planes, and circular and elliptical contoured apertures only have −17 dB side lobe levels in the principal E and H planes; such side lobe levels in the transmit pattern requires ultra-low (and difficult to realize) side lobe levels in the receive pattern to achieve the currently required −50 dBc 2-way side lobe levels.
Side lobe level is an important performance requirement for AESAs. Typically, the first side lobe level is 13.5 dB below the peak of the main beam when operating with uniform illumination; however, in some applications, lower side lobe levels are required. One scheme currently used to reduce side lobe levels is to taper the radiated power from each element across the AESA such that the elements in the middle of the AESA are radiated with the highest power and the rest of the elements are symmetrically tapered. Currently, tapering the output power is achieved by feeding each transmitter with different RF power input levels; however, the transmitter efficiency drops significantly as the power amplifier output power is backed off from the peak power. Additionally, a common approach to taper output power is to use different transmit/receive modules with each transmit/receive module designed for a different peak power. In such an approach, each of the transmitters is operated at saturation for maximum efficiency but also operated at different power levels. For example, if three different power levels are needed across the AESA, the AESA is implemented with three different transmit/receive module types, each with different transmitted powers; however, this requires three times as any many parts, which significantly increases both recurring and non-recurring costs for the AESA.
Therefore, it would be desirable to provide a method, apparatus, and system which achieves power tapering across the AESA, achieves high power added efficiency, and reduces power amplifier amplitude quantization, which in turn reduces deterministic peak side lobe levels in the AESA's far field radiation pattern.